Injection of the anesthesia in the dental treatment has been carried out by using various cartridge-type motorized syringes in recent years. One of them was developed by assignors of this application and has been put widely to practical use. Reference 1 can be made to Japanese patent application directed to such a cartridge-type motorized syringe and published under Publication 2001-70444. The cartridge-type motorized syringe comprises an electric motor with reduction gears, a pinion adapted to be rotated through a planetary reduction gears by the motor, and a plunger rod having a rack meshed with the pinion to move forwardly the plunger rod from its initial retracted position, thereby pushing a plunger rubber fitted in an anesthetic filled cartridge so that the anesthetic can be ejected from the cartridge through a needle attached to the cartridge while piercing a rubber plug fitted in the cartridge at its tip. The forward movement of the plunger rod for the injection of anesthetic into the oral cavity is performed at extremely low speeds such as 30 mm/60 seconds, 30 mm/100 seconds and 30 mm/200 seconds, for example. Upon completion of the injection, the plunger rod is required to return to the initial retracted position. This can be achieved by pushing the plunger rod back to the initial retracted position quickly by the hand of a dental surgeon after the operative connection between the pinion meshed with the rack on the plunger rod and the drive mechanisms is cut off.
There were two problems to be solved in this motorized dental syringe. One of them was cracking of the glass tube wall of the cartridge by contact of the tip of the plunger rod with the inner surface of the glass tube wall, due to offset of the plunger rod from the center of the rubber plunger in the cartridge while the plunger rod pushes the rubber plunger with the tip end thereof. The inventors of this application found that one of the causes of the cracks lay in the rack and pinion mechanism incorporated in the syringe. Since force of 300 N (about 30 kg) is exerted on the plunger rod during injection to eject the anesthetic during injection, the geared motor deviates in a direction escaping from the load imposed on the motor so that some force is applied to the rack on the plunger rod to maintain the rack relative to the pinion driven by the motor at 90 degrees. As a result, the tip of the plunger rod deviates from its given straight position. Namely, the tip of the plunger rod contacts the rubber plunger in the cartridge at a position deviated from the center of the rubber plunger. As the other problem, the above syringe had disadvantages of complicating the drive mechanism for the rack and pinion because of use of the planetary reduction gears as a reduction gear, and that the means for cutting off the operative connection between the pinion and the drive mechanism was also complicated in structure.